Method of controlling an etching process

ABSTRACT

In an etching process, the degree of etching of a workpiece is controlled by scanning a segment of the workpiece at an intermediate point in the process to derive a measure of the portion of the segment from which a surface layer has been removed. A variable parameter of the etching process, such as the conveyor speed or the etchant flow rate, is adjusted, according to the difference between the derived measure and a standard measure, in a direction tending to reduce the difference.

[451 Apr. 30, 1974 United States Patent [191 Brown METHOD OF CONTROLLINGAN ETCHING PROCESS [75] Inventor:

Primary Examiner-William A. Powell Attorney, Agent, or Firm-B. W.Sheffield Martin John Brown, Lambertville, NJ.

[73] Assignee: Western Electric Company,

ABSTRACT Incorporated, New York, N .Y.

Nov. 24, 1972 Appl. No.: 309,378

In an etching process, the degree of etching of a workpiece iscontrolled by scanning a segment of the work- [22] Filed:

piece at an intermediate point in the process to derive a measure of theportion of the segment from which a surface layer has been removed. Avariable parameter of the etching process, such as the conveyor speed orthe etchant flow rate, is adjusted, according to the difference betweenthe derived measure and a standard 525 404 3 3 w a 5W 1 3 s w w 5 m l l..n a m 3 e a 6 u m 5 n 1 mmfim L f C WM st e Umm 11]] 2 8 555 [[lmeasure, in a direction tending to reduce the difference.

References Cited UNITED STATES PATENTS 3,702,277 156/345 14 9 D'awmgF'gures W D m E F P s T R N A m H E m w E o C an RU TC mm C m mu LN DE RP E L F E R PATENTEHAPR 30 m4 sum 1 or 4 wIII k wozwmwmm H53 SBmdwmETCHANT PATENTED APR 30 I974 CONTROL CIRCUIT PATENIEDAPR 3o IIIIIREFERENCE SHEU 3 III-4 50 I 25 "*gg i SIGNAL I LEvEL NT DISCRIMINATOR ERI 53 1'\ pgNsTAtljTg I R P L E 30 GENERATOR 76 Q' P 56 I [Fa i*SEQUENCER v5? ENABLE COMPARATOR l 28 l DIGITAL I To INITIAL T T 66\ 70I ANALOG AMPLIFIER VALUE 7| 7 X CONVERTER 3 I DECREMENTI INCREMENTINITIALIzE l I s7- ACCUMULATOR I POwER SOURCE 5 TO PUMP f TO CONVEYORMOTOR l7 MOTOR 24 5 ;;9 INPUT FROM E 90 '45 5 SENSOR 25 g I OUTPUT TO IAND-GATE 52 I I i i fEBE'! i I m m l l 1 UPPER 9| E ,REFERENCE I 1 93 ii OUTPUT INPUT FROM 1 1 To SENSOR 25 I AND-GATE LOwER j i METHOD OFCONTROLLING AN ETCHING PROCESS BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention relates to methods and apparatus forcontrolling a spray process for the treatment of workpieces, and moreparticularly, to methods and apparatus for regulating a spray-etchingmachine according to an analysis of the light reflected from a segmentof the surface of a workpiece being etched.

2. Description of the Prior Art One step in the fabrication of amulti-layered, patterned workpiece, for example, an etched circuitboard, is the removal by etching of unwanted portions of a surface layerof the workpiece to produce a desired pattern in the surface layer.Typically, predetermined regions of the workpiece are protected from theetchant by an etch-resistant layer called a resist. The workpiece isimmersed in a bath, or conveyed through a spray, of a liquid etchant toremove those portions of the surface layer that are not protected by theresist. After etching is completed, the resist is removed.

An etchant spray typically etches faster than an etchant bath, becausethe spray constantly projects fresh etchant onto the surface beingetched. Therefore, an etching machine preferably includes means forconveying workpieces past one or more spray nozzles, and meansfor'recirculating etchant from a sump beneath the conveying means to thespray nozzles.

A properly etched workpiece is neither underetched nor overetched. Theworkpiece is underetched if the etchant does not remove all unwantedportions of the surface layer of the workpiece. Conversely, theworkpiece is overetched if the etchant undercuts the resist and removespart of the desired pattern. The degree of etching must be carefullyregulated to prevent either underetching or overetching. In aspray-etching machine the degree of etching is typically regulated byvarying the rate of etchant flow, and/or by varying the conveyor speed.A machine operator usually adjusts these variables manually, accordingto his visual inspection of etched workpieces leaving the sprayetchingmachine.

In a typical spray-etching machine, a batch of etchant is constantlyrecirculated from the sump to the spray nozzles. The chemical activityof the etchantbatch decreases as it becomes contaminated with thematerial being etched. Certain etchants also lose strength with time.Means can be provided to regenerate or replenish the etchant, but suchmeans do not compensate completely for variations in etchant strength.

Variations in properties of the surface layer can also effect the degreeof etching. For example, a copper surface layer can vary in thicknessand in hardness. Different batches of copper can etch at significantlydifferent rates because of such variations. Typically, it has heretofore been necessary for the machine operator to readjust either theetchant flow or the conveyor speed to conpensate forthe variations inetchant strength and workpiece properties.

Obviously, it would be desirable to automatically control the degree ofetching in a spray-etching machine without human intervention. To thisend, it becomes necessary to generate a control signal that representsthe actual degree of etching of the workpiece.

This control signal may then be used to regulate a variable parameter inthe etching machine to achieve the desired degree of etching. Variousmeans have been proposed for generating such a control signal. Forexample, it is known to sense light that is reflected or transmitted bya workpiece being etched, and to terminat e the etching process when thelight reaches a certain intensity. However, it has been found difficultto adapt such light-sensing means to conveyorized sprayetching machinesbecause of fluctuations in the intensity of the light caused by theopacity and reflectivity of the etchant. Such fluctuations in lightintensity limit the area of the workpiece that can be satisfactorilysensed. In my invention, however, I disclose methods and apparatus thatutilize scanning techniques for automatically controlling the degree ofetching of workpieces in a spray-etching machine.

SUMMARY OF THE INVENTION According to the invention, a workpiece havinga portion whose area is altered during etching is conveyed through aspray-etching machine having a vari-' able process parameter. A segmentof the workpiece is Y scanned, at a location in the etching machinewhere the portions are being altered in area, to derive an electricalsignal that represents the altered portion of the segment. The derivedelectrical signal is then compared with a predetermined electricalsignal that represents the desired area of the altered portion at thescanning 7 location, the variable process parameter is adjusted, ac-

cording to. the difference between the derived and the predeterminedelectrical signals, in a direction that tends to reduce the difference,thereby regulating the degree of etching.

In a first, preferred embodiment, the variable process parameter is therate of etchant flow. In a second embodiment, the variable processparameter is the speed at which the workpiece is conveyed through theetching process.

The scanning and comparison steps are accomplished with areflected-light sensor and a control cit"- cuit. Alternate embodimentsof the sensor and the control circuit are disclosed for use withworkpieces having various reflectance characteristics, and forcontinuous workpieces. I

These and other features of the invention will be more fully understoodfrom a consideration of the attached drawings and the followingdescription of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a portion of a workpiecewhich can be fabricated using the methods and apparatus of theinvention;

FIG. 2 is a partly schematic, partly diagrammatic representation of aspray-etching machine for use with discrete workpieces, which may becontrolled by the method and apparatus of the invention;

FIG. 3 is a partly schematic, partly diagrammatic representation of aspray-etching machine for use with a continuous workpiece, which may becontrolled by the method and apparatus of the invention;

FIG. 4 is a more detailed diagram of the reflected light sensor shown inFIGS. 2 and 3;

FIG. 5 is a more detailed diagram of a first, preferred, embodiment of acontrol circuit shown in FIGS. 2 and FIGS. 6 and 7 are more detaileddiagrams of a signal level discriminator shown in FIGS. and 8;

FIG. 8 is a more detailed diagram of a second embodiment of the controlcircuit shown in FIGS. 2 and 3; and

FIG. 9 shows a modification of the control circuits shown in FIGS. 5 and8 that may be used with the continuous workpiece shown in FIG. 3.

DETAILED DESCRIPTION Throughout the following description, identicalreference numerals are used to identify identical elements in differentfigures.

FIG. 1 shows a portion of a workpiece 10 that can be fabricated usingthe instant invention. The workpiece 10 is comprised of a substrate 11having pattern elements, such as elements 12 and 13 thereon, which areformed by etching away unwanted portions of a surface layer laminated tothe substrate 11. A well-known example of such a workpiece is an etchedcircuit board in which the substrate 11 is an insulating material, suchas polyester, phenolic resin, epoxy glass, or the like, and the patternelements 12 and 13 are a conducting material, such as copper. During thefabrication of such a workpiece, the pattern elements are protected fromthe etchant used in the etching process by a layer of resist (notshown).

It is sometimes desirable to fabricate double-sided workpieces havingpattern elements on both sides of a substrate. Pattern elements 14 and15 are indicated to illustrate such a double-sided workpiece. It isselfevident that the instant invention may be used to fabricatedouble-sided workpieces as well as single-sided workpieces; however, theinvention will be described only for use with single-sided workpieces,unless otherwise stated.

Referring now to FIG. 2, the workpieces 10 are carried by a conveyor 18,which is driven by a motor 17, through an etching zone 20, whichcomprises a plurality of etchant sprays 21. The workpieces 10 can beconveyed horizontally, as shown, or in any other convenient orientation.Etchant from the sprays 21 is collected in a sump 22. An etchant pump23, which is driven by a motor 24, circulates the etchant from the sump22 to the sprays 21.

A sensor 25 scans a passing workpiece 10 by illuminating a region 26 ofthe workpiece, responding to the light that is reflected from the region26, and transmitting an analog signal, which represents the magnitude ofthe reflected light, to a control circuit 28. A presence sensor 30proximate the sensor 25 transmits a digital signal, which is generatedby photodetecting means or the like, to the control circuit 28 when oneof the workpieces 10 is within the scanning range of the sensor 25. Forclarity, the workpiece 10 that is within scanning range of the sensor 26will be designated as scanned workpiece 16. Each of the workpieces 10becomes the scanned workpiece 16, in turn, as it is conveyed past thesensors 25 and 30.

The control circuit 28 processes the reflected light signal from thesensor 25 to determine the degree of etching of the scanned workpiece16. In a first preferred embodiment, the control circuit 28 thenregulates the degree of etching by varying the rate of etchant flow,with a conveyor speed held constant. In a second embodiment, the controlcircuit 28 then regulates the degree of etching by varying the conveyorspeed,

with the rate of etchant flow held constant. The control circuit 28 willbe explained more fully in the descriptions of FIGS. 5 and 8.

The workpieces 10 have been shown and described as discrete workpieces.However, it will be clear that the invention can also be used tofabricate continuous, web-like workpieces, such as a continuousworkpiece 35, shown in FIG. 3, which may include repeated patternelements 36. The continuous workpiece is moved from supply means totakeup means (both not shown) and through the etching zone 20 by themotor 17 in an anlogous manner to the movement of the discreteworkpieces 10 in FIG. 2.

FIG. 4 is a diagrammatic representation of the sensor 25. A lamp 40 isplaced near the end of an outer light pipe 41, which conducts light fromthe lamp 40 to illuminate the region 26 of the scanned workpiece 16. Aninner light pipe 43 is mounted concentrically with a portion of theouter light pipe 41 to conduct light reflected from the region 26 to thephotodetector 44. The light pipes 41 and 43 are preferably fabricatedfrom fused silica (quartz) for maximum light transmission. The innerlight pipe 43 is coated with a thin, reflecting film to minimize thedirect passage of light from the outer light pipe 41 to the inner lightpipe 43. The annular space between light pipes 41 and 43 is filled withan etchant resistant epoxy resin to prevent etchant from penetratingbetween the light pipes. The sensor 25 is enclosed within a housing (notshown) which provides means for mounting the sensor at the correctposition with respect to the scanned workpiece 16. A circuit 46, whichcan include signal amplifying and conditioning means, prepares theoutput signal from the photodetector 44 for transmission as an analogsignal to the control circuit 28.

The sensor 25 can be mounted in the path of the etchant sprays 21, asshown in FIG. 2, or in an area protected from etchant. Some etchants,such as ferric chloride, transmit little light; therefore, it ispreferable to prevent such etchants from intervening between the sensor25 and the scanned workpiece 16. Advantageously, the sensor 25 ismounted close to the scanned workpiece 16 with the concentric ends ofthe light pipes 41 and 43 positioned about 0.125 inch from the surfaceof the workpiece. A deflector (not shown) can also be added to directetchant away from the sensor.

As the scanned workpiece 16 is carried past the sensor 25, the region 26moves with respect to the workpiece, so that the sensor 25 scans asegment 45 of the workpiece. The sensor 25 is positioned so that thepattern in the segment 45 is representative of the pattern on theworkpiece as a whole.

The surface of one of the workpieces 10 typically comprises three typesof areas: (I incompletely etched surface layer areas, (2) resist areasand (3) exposed substrate areas. A surface layer area becomes an exposedsubstrate area when the overlying surface layer is completely etchedaway. Because these three types of areas typically have differentreflectivity characteristics, the sensor 25 can usually be constructedto provide an analog output signal that is within a different range foreach type of area.

EXAMPLE 1 A first sensor 25 was constructed to sense an etched circuitcard that comprised a light-colored synthetic substrate, a coppersurface layer and a solder resist. In this sensor, the lamp 40 was anincandescent lamp operated at low power to enhance red emission, and thephotodetector 44 was a Texas Instruments LS-400 photodiode. Since copperreflects red light optimally, compared to the light-colored substrate orthe solder resist, and since the LS-40O photodiode is particularlysensitive to red light, the analog output signal from the sensor waswithin a maximum range when the region 26 was copper, and was withinlesser ranges when the region 26 was exposed substrate or the resist.Alternatively, if the incandescent lamp had been operated at full power,a red filter could have been placed in the optical path of the sensor25, for example, between the lamp and the light pipe 41.

EXAMPLE 2 A second sensor 25 was constructed to sense a memory card thatcomprised a highly reflective aluminum substrate, a poorly reflectivemagnetic alloy surface layer, and a slightly reflective resist. In thesecond sensor 25 the lamp 40 was an incandescent lamp operated at fullpower, and the photodetector 44 was again a Texas Instruments LS-40Ophotodiode. The analog output signal from the sensor 26 was within amaximum range when. the region 26 was a substrate area, a medium rangewhen the region 26 was a resist area, and a minimum range when theregion26 was a magnetic alloy area.

Referring briefly to FIG. 2, the sensor 25 is positioned at a locationin the etching zone 20 where some parts of the unprotected regions onthe scanned workpiece 16 are exposed substrate, and other parts of theunprotected regions are incompletely etched surface layer. At thislocation, the areas of the incompletely etched surface layer parts andthe exposed substrate parts are being altered by the etching process.The proportion of surface layer area to' exposed substrate area in theunprotected regions is an indication of the degree of etching. If thescanned workpiece 16 is being etched at too fast a rate, a relativelylarger proportion of the unprotected regions will be exposed substrate;conversely, if the scanned workpiece 16 is being etched at too slow arate, a relatively smaller proportion of the unprotected regions will beexposed substrate. The degree of etching can be detennined, therefore,by measuring either the area of exposed substrate or the area ofincompletely etched surface layer in the segment 45 of the scannedworkpiece 16 at the scanning location.

FIG. 5 is a more detailed block diagram of a first, preferred,embodiment of the control circuit 28 shown in FIG. 2. Referring to FIG.5, the signal from the sensor 25 is connected to the input of asignal-level discriminator 50. The output of the signal leveldiscriminator is connected to the first input of an AND-gate 52, and theoutput of a constant-rate pulse generator 53 is connected to a secondinput of the AND-gate 52. The output of the AND-gate 52 is connected tothe count input of a counter 55. The output of the counter 55 isconnected by a data path 56 to a first input of a comparator 57. Astandard count is stored in a switch register, or other means (notshown) that is connected by a data path 61 to a second input of thecomparator 57. A first output of the comparator 57, which is activatedwhen the comparator'is enabled and the accumulated count in the counter56 is less than the standard count,

is connected by the data path 66 to a decremenf input of an accumulator67. A second output of the comparator 57, which is activated when .thecomparator is enabled and the accumulated count in the counter 56 isgreater than the standard count, is connected by the data path 67 to anincrement input of the accumulator 67. An initializing means (not shown)is connected by a data path 71 into the accumulator 67. The output ofthe accumulator 67 is connected by a data path 72 to a digital-to-analog(D/A) converter 73. The output of the D/A converter is connected to anamplifier 75, which, in the preferred embodiment, is connected to drivethe pump motor 24. The presence detector 30 is connected to a thirdinput of the AND- gate 52, and to the input of a sequencer 76. First andsecond outputs of the sequencer 76 are connected to an enable" input ofthe comparator 57 and a reset input of the counter 55, respectively. Apower source 77 is connected to the conveyor motor 17.

It will be necessary to refer to digital logic levels in the remainingdescription. For brevity, a zero logic level will be designated as 0,and a one logic level will be designated as l.

The signal level discriminator 50 compares the analog signal from thesensor 25 with one or more reference levels that are chosen to definethe range within which the analog signal represents the type of surfacearea to be measured. The output from the signal level discriminator is 1when the analog signal is within the defined range, and 0 when theanalog signal is outside the defined range.

When the defined range is at the upper or lower end of the analog signalrange, the signal level discriminator 50 can be a well-known analogcomparator 90, as shown in FIG. 6. When the defined range is in themiddle of the analog signal range, the signal level comparator 50 can bea combination of two comparators 91 and 92, as shown in FIG. 7.

In FIG. 6, the analog signal on the input lead is connected to theterminal of the comparator 90, and a reference signal is connected tothe terminal of the comparator. The output of the comparator is then 0when the input signal is less than the reference signal, and 1 when theinput signal is greater than the reference signal. Alternatively, if thereference signal were connected to the terminal of the comparator andthe input signal were connected to the terminal of the comparator, theoutput of the comparator would be 0 when the input signal is greaterthan the reference signal, and 1 when the input signal is less than thereference signal.

For use with the workpiece described in Example 1, the comparator 90shown in FIG. 5 can be used for the signal level discriminator 50. Thereference level is set near the lower end of the maximum range withinwhich the analog signal from the sensor 25 indicates that the region 26(FIG. 4) is copper. Thus, the output of the comparator 90 is 1 when theregion 26 is copper, and 0 otherwise.

For use with the workpiece described in Example 2, the comparator 90 canalso be used, but with the reference level connected to the terminal ofthe comparator and the input connected to the terminal of thecomparator. The reference level is set near the upper end of the rangewithin which the analog signal from the sensor 25 indicates that theregion 26 is magnetic alloy. Thus, the output of the comparator 90 is 1when the region 26 is magnetic alloy, and otherwise.

In FIG. 7, the analog signal on the input lead is connected to,theterminal of the comparator 91 and the terminal of the comparator 92. Anupper reference level is connected to the terminal of the comparator 91,and a lower reference level is connected to the terminal of thecomparator 92. The outputs of the comparators 91 and 92 are connected tothe inputs of an AND-gate 93.

When the analog signal is less than the lower reference level, theoutputs of the comparators 91 and 92 are l and 0, respectively, so thatthe output of the AND-gate 93 is 0. When the analog signal is betweenthe lower and the upper reference levels, the outputs of bothcomparators 91 and 92 are l, and the output of the AND-gate 93 is 1.When the analog signal is greater than the upper reference level, theoutputs of the comparators 91 and 92 are 0 and 1, respectively, so thatthe output of the AND-gate 93 is again zero.

It will be clear that it is also possible, and may be desirable, toconstruct a particular sensor 25 and a particular signal leveldiscriminator 50 so that the output of the discriminator 50 is 1 whenthe area 26 is exposed substrate, instead of incompletely etched surfacelayer.

Referring again to FIG. 5, in the operation of the first embodiment ofthe control circuit 28, the power supply 77 drives the conveyor motor 17at an essentially constant speed. The presence sensor 30 applies 1 toboth the AND-gate 52 and the sequencer 76 when the scanned workpiece 16is within range of the sensor 25, and 0 to both the AND-gate 52 and thesequencer 76 otherwise. For this explanation, assume that the sensor 25and the signal level discriminator 50 are constructed so that the signallevel discriminator 50 applies 1 to the AND-gate 52 when the region 26is incompletely etched surface layer. The constant-rate pulse generator53 applies periodically repeated 1 pulses to the AND- gate 52.

As the scanned workpiece 16 (FIG. 2) passes the sensors 25 and 30, thesensor 25 scans the segment 45 (FIG. 4) and provides a varying analogsignal to the signal level discriminator 50. When the region 26 isincompletely etched surface layer, the signal level discriminatorapplies 1 to the AND-gate 52. When the inputs to the AND-gate 52 fromthe signal level discriminator 50 and the sensor 30 are both I, theAND-gate 52 enables pulses from the pulse generator 53 to reach thecount input of the counter, Since the scanned workpiece 16 is moved atan essentially constant speed, and the repetition rate of the pulsesfrom the pulse generator 53 is constant, the accumulated count in thecounter 55 represents the area of the scanned strip 45 (FIG. 3) that isincompletely etched surface layer.

It will be clear that the workpieces must be guided with respect to thesensor by means not shown so that the same portion of the pattern oneach scanned workpiece 16 is scanned by the sensor 25.

The standard count is the value that would accumulate in the counter 55if the scanned workpiece 16 were etched to the desired degree. If thescanned workpiece 16 is overetched, the accumulated count therefor isless than the standard count, because a lesser area of the scanned strip45 thereon is incompletely etched surface layer. Conversely, if thescanned workpiece 16 is underetched, the accumulated count therefor isgreater than the standard count, because a greater area of the scannedstrip 45 thereon is incompletely etched surface layer.

When the trailing edge of the scanned workpiece 16 passes the presencesensor 30, the signal from the presence sensor changes from 1 to 0. Thischange triggers the sequencer 76 to apply a first pulse to the enableinput of the comparator 57 followed by a second pulse to the reset inputof the counter 55. The comparator 57, when enabled, compares theaccumulated count in the counter 55 with the standard count, andactivates either an output on the data path 66 to decrement theaccumulator 67, if the accumulated count is less than the standardvalue, or an output on the data path to increment the accumulator 67, ifthe accumulated count is greater than the standard count. The secondpulse from the sequencer 76 resets the counter 55 to an initial count,which is typically zero.

The signals transmitted over the data paths 66 and 70 to the accumulator67 can indicate the magnitude, as well as the sign, of the differencebetween the accumulated count and the standard count, and theaccumulator can be incremented or decremented by the magnitude of thedifference. Thus, a large difference between the accumulated count andthe standard count, indicating a wide variation between the actualdegree of etching of the scanned workpiece and the desired degree ofetching thereof, results in a large change in the contents of theaccumulator 67; conversely, a small difference results in aproportionately smaller change. Alternatively, the signals transmittedover the data paths 66 and 70 can increment or decrement the accumulator67 by fixed amounts that are not related to the actual differencebetween the accumulated count and the standard count.

An initial value is set into the accumulator 67 over the data path 71 toestablish an initial etchant flow when the spray etching machine isfirst started. The initial value is typically estimated by consideringthe amount of material that is to be etched from the workpieces and theknown strength of the etchant.

The digital value stored in the accumulator 67 is converted to an analogvalue by the D/A converter 73. The analog signal is then amplified bythe amplifier 75 to drive the etchant pump 23. The value in theaccumulator 67 thus represents the rate of etchant flow.

The resulting operation of the first embodiment of the control circuit28 is such that the rate of etchant flow is decreased when the scannedworkpiece 16 is overetched, as indicated by the accumulated counttherefor being less than the standard count; and the rate of etchantflow is increased when the workpiece is underetched, as indicated by theaccumulated count therefor being greater than the standard count.

A second embodiment of the control circuit 28 is shown in FIG. 8, whichis generally similar to FIG. 5.

However, as shown in FIG. 8, the first output of the comparator 57 isconnected by a data path 81 to the increment input of the accumulator67, and the second output of the comparator 57 is connected by a datapath 82 to the decrement input of the accumulator 67. The output of theD/A converter 73 is connected to an amplifier 76 which is furtherconnected to the conveyor motor 17. A power source 78 is connected tothe pump motor 24. The output of the amplifier 76 is also connected to avariable-rate pulse generator 54.

In the operation of the second embodiment, the degree of etching iscontrolled by varying the conveyor speed, and holding the etchant flowconstant. The value in the accumulator 67 thus represents the conveyorspeed. The power source 78 drives the pump motor 24 to maintain asubstantially constant rate of etchant flow, and the amplifier 76 drivesthe conveyor motor 17 and regulates the repetition rate of the pulsegenerator 54 to be proportional to the speed of the conveyor motor 17.Thus, the number of pulses generated by the pulse generator 54 is thesame for each workpiece, regardless of the speed of the conveyor, andthe count accumulated by the counter 55 is independent of the conveyorspeed.

The comparator 57 activates either an output on the data path 81, toincrement the accumulator 67 when the accumulated count in the counter55 is less than the standard count; or an output on the data path 82, toincrement the accumulator 67 when the accumulated count in the counter55 is greater than the standard count. Again, the accumulator 67 can beincremented or decremented by either the magnitude of the differencebetween the accumulated count and the standard count, or a fixed amount.

The resulting operation of the second embodiment of the control circuit28 is such that the conveyor speed is increased when the scannedworkpiece 16 is overetched, as indicated by the accumulated counttherefor being less than the standard count; and the conveyor speed isdecreased when the scanned workpiece 16 is underetched, as indicated bythe accumulated count therefor being greater than the standard count.

If it is desired to construct the sensor 25 and the signal leveldiscriminator 50 so that the output of discriminator 50 is I when thearea 26 is exposed substrate, instead of incompletely etched circuitlayer, the connections between the comparator 57 and the accumulator 67must be the reverse of those described above. In this situation, if thespray-etching machine is to be regulated by varying the etchant flow,the first embodiment of the control circuit 28, as shown in FIG. 5, isused; however, the comparator 57 is connected to the accumulator 67 bythe data paths 81 and 82, as shown in FIG. 8. Similarly, if thespray-etching machine is to be regulated by varying the conveyor speed,the second embodiment of the control circuit 28, as shown in FIG. 8, isused; however, the comparator 57 is connected to the accumulator 67 bythe data paths 66 and 70, as shown in FIG. 5.

Referring again to FIG. 3, the workpiece can be replaced, as explainedpreviously, by a continuous workpiece 35. The presence sensor 30 canrespond to index marks (not shown) on the continuous workpiece thatcorrespond to each appearance of therepeated pattern, so that similarworkpiece areas are repeatedly scanned. Alternatively, the presencesensor 30 can be replaced by a sampling clock 81, in the control circuit28, as shown in FIG. 9. The clock 81 can be used with either the firstembodiment of the control circuit 28, shown in FIG. 5, or the secondembodiment of the control circuit 28, shown in FIG. 8. The clock 81generates periodic sampling pulses, which simulate the signal that isreceived from the presence detector 30 when either discrete workpiecesare etched or index marks are used on the continuous workpiece. However,the 'clock 81 can be used only when the pattern on the continuousworkpiece is homogeneous, so that each resulting sampled area of thecontinuous workpiece comprises essentially the same mix of the threetypes of surface areas, and a standard count can be established for atypical sampled area.

If it is desired to etch both sides of workpieces 10 in the apparatus ofFIG. 2, a second set of sprays (not shown) can be used in conjunctionwith a suitable conveyor to etch the second side of the workpiece 10opposite the first side etched by etchant from the sprays 21.Preferably, the degree of etching of the second side is controlledseparately from that of the first side. The conveyor speed is obviouslycommon to both sides of the workpieces, so this variable is notconvenient for such separate control. Thus, the rate of etchant flow isappropriately used as the variable for control, requiring the additionof a pump, pump motor, sensors and a control circuit (all not shown) forthe second set of sprays. Analogously, both sides of a continuousworkpiece, as shown in FIG. 3, can be etched.

The elements of both embodiments of control circuit 28 are all wellknown in the art. The counter 55 is preferably a binary counter, and thecomparator 57, accumulator 67, and D/A converter 73 are also preferablybinary devices. The amplifiers 75 and 76 are conventionally designed todrive the pump motor 24 and the conveyor motor 17, respectively. Manypossible configurations of such an amplifier are known in the art. Thecomparators shown in FIGS. 6 and 7 are again well known. The operationand application of elements such as those shown in FIGS. 5-9, inclusive,are described in greater detail in Arithmetic Operations in DigitalComputers, by R. K. Richards, D. Van Nostrand Company, Inc., 1955.

In the following claims, a properly etched workpiece is a workpiece thatis neither overetched nor underetched.

One skilled in the art may make changes and modifications to theembodiments of the invention disclosed herein, and may devise otherembodiments, without departing from the spirit and scope of theinvention.

What is claimed is: l. A method of regulating the degree of etching of aworkpiece being conveyed through an etching process, the etching processhaving at least one variable process parameter that affects the degreeof etching, the workpiece having a portion whose area is altered duringetching, which comprises:

scanning a segment of the workpiece, at a location in the etchingprocess where the portion is being altered, to derive a signalrepresentating the area of the altered portion within the segment;

comparing the derived signal with a predetermined signal representingthe desired area of the altered portion at the scanning location; and

adjusting a variable process parameter, according to the result of thecomparing step, in a direction that tends to reduce the differencebetween the derived signal and the predetermined signal, to regulate thedegree of etching.

2. The method of claim 1 wherein the scanning step further comprises:

directing a beam of light onto the segment as the segment passes thescanning location;

generating an electrical signal directly related to the intensity of thelight reflected from the segment;

generating electrical pulses at a repetition rate proportional to thespeed at which the workpiece is conveyed past the scanning location; and

counting the electrical pulses when the reflected light signal indicatesthat the light beam is reflected from the altered portion, to therebygenerate the derived signal.

3. The method of claim 1 wherein the altered portion is a surface layerthat decreases in area during etching, the variable process parameter isthe rate of etchant flow, and the adjusting step further comprises:

increasing the rate of etchant flow when the derived signal is greaterthan the predetermined signal, and

decreasing the rate of etchant flow when the derived signal is less thanthe predetermined signal.

4. The method of claim 1 wherein the altered portion is a surface layerthat decreases in area during etching, the variable process parameter isthe speed at which the workpiece is conveyed through the etching processand the adjusting step further comprises:

increasing the conveying speed when the derived signal is less than thepredetermined signal, and decreasing the conveying speed when thederived signal is greater than the predetermined signal.

5. The method of claim 1 wherein the altered portion is exposedsubstrate that increases in area during etching, the variable processparameter is the rate of etchant flow, and the adjusting step furthercomprises:

increasing the rate of etchant flow when the derived signal is less thanthe predetermined signal, and decreasing the rate of etchant flow whenthe derived signal is greater than the predetermined signal.

6. The method of claim 1 wherein the altered portion is exposedsubstrate that increases in area during etching, the variable processparameter is the speed at which the workpiece is conveyed through theetching process and the adjusting step further comprises:

increasing the conveying speed when the derived signal is greater thanthe predetermined signal, and decreasing the conveying speed when thederived signal is less than the predetermined signal.

7. In an etching process, a method of determining the degree of etchingof a workpiece having a surface layer portion that is decreased in areaduring etching, which comprises:

scanning a segment of the workpiece, at a location in the etchingprocess where the surface layer portion is being decreased in area, toderive a signal representing the area of the surface layer portionwithin the segment; and

comparing the derived signal with a predetermined signal representingthe desired area of the surface layer portion at the scanning locationto indicate that, at the scanning location, the workpiece is underetchedwhen the derived signal is greater than the predetermined signal,properly etched when the derived signal substantially equals thepredetermined signal, and overetched when the derived signal is lessthan the predetermined signal.

8. In an etching process, a method of determining the degree of etchingof a workpiece having an exposed substrate portion that is increased inarea during etching, which comprises:

scanning a segment of the workpiece, at a location in the etchingprocess where the exposed substrate portion is being increased in area,to derive a signal 9. Apparatus for regulating the degree of etching ofa workpiece conveyed through an etching process having at least onevariable process parameter affecting the degree of etching, theworkpiece having a portion whose area is altered during etching, whichcomprises:

means for scanning a segment of the workpiece, at a location in theetching process where the portion is being altered, to determine a valuerepresenting the area of the altered portion within the segment;

means for comparing the determined value with a standard value for thesegment; and

means for adjusting a process parameter, according to the output of thecomparison means, in a direction tending to reduce the differencebetween the determined value and the standard value.

10. The apparatus of claim 9 in which the workpiece is conveyed throughthe etching machine at a constant speed, the variable process parameteris the rate of etchant flow, and the means for scanning comprises:

photodetector means, positioned at the scanning location to receivelight reflected from the segment,

for generating an electrical signal directly related to the intensity ofthe reflected light;

a discriminator connected to the photodetector for generating an outputsignal when the signal from the photodetector is within a rangeindicating that the light is reflected from the altered portion of thesegment;

a pulse generator for generating pulses at a constant repetition rate;and

a counter, connected to both the pulse generator and the discriminator,for counting the pulses from the pulse generator when the output signalfrom the discriminator is present, the count accumulated in the counterbeing the value representing the area of the altered portion within thesegment.

11. The apparatus of claim 9 in which the rate of etchant flow in theetching process is constant, the variable process parameter is the speedat which the workpiece is conveyed through the etching process, and themeans for scanning comprises:

photodetector means, positioned at the scanning location to receivelight reflected from the segment, for generating an electrical signaldirectly related to the intensity of the reflected light;

a discriminator connected to the photodetector for activating an outputsignal when the signal from the photodetector is within a rangeindicating that the light is reflected from the altered portion of thesegment; means for generating pulses at a repetition rate proportionalto the conveying speed; and

a counter,. connected to both the pulse generating means and thediscriminator, for counting the pulses from the pulse generating meanswhen the output signal from the discriminator is present, the countaccumulated in the counter being the value representing the area of thealtered portion within the segment.

12. Apparatus for determining the degree of etching of a workpiece in anetching process, the workpiece having a surface layer portion that isdecreased in area during etching, which comprises:

means for scanning a segment of the workpiece, at a location in theetching process where the surface layer portion is being decreased inarea, to determine a value representing the area of the surface layerportion within the segment;

means for comparing the determined value with a standard value for thesegment; and

means for indicating that the workpiece is overetched when thedetermined value is less than the standard value, properly etched whenthe determined value substantially equals the standard value, andunderetched when the determined value is greater than the standardvalue.

13. Apparatus for determining the degree of etching of a workpiece in anetching process, the workpiece having an exposed substrate portion thatis increased in area during etching, which comprises:

means for scanning a segment of the workpiece, at a location in theetching process where the exposed substrate portion is being increasedin area, to determine a value representing the area of the exposedsubstrate portion within the segment;

means for comparing the determined value with a standard value for thesegment; and

means for indicating that the workpiece is overetched when thedetermined value is greater than the standard value, properly etchedwhen the determined value substantially equals the standard value, andunderetched when the determined value is less than the standard value.

14. A method of controlling the selective removal of a layer of materialfrom a workpiece comprising the steps of:

subjecting the workpiece to a variable rate etching process,

scanning a segment of the workpiece at an intermediate point in theprocess to derive a measure of the portion of the segment from which thematerial has been removed.

comparing the derived measure with a standard measure representing thedesired amount of material removal at said intermediate point, and

varying the etching rate in accordance with the comparison atsaid'intermediate point.

L-566-PT UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 2.808.067 Dated April 30, 197M lnventor(s) Martin John Brown It IScertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

[- In the specification, Column 1, line 55, effect" should l read--affect--. Column 5, line 11, "was exposed should i read --was theexposed". Column 7, line 50, "the count input" should read --the "count"input-H Column 8, lines 7-8, "the enable input" should read -the enable"input---; line 9, "the reset input" should read --the "reset" input--.

In th l Column line n m r Should read --removed,--. I

Signed and sealed this 24th day of September 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents

2. The method of claim 1 wherein the scanning step further comprises:directing a beam of light onto the segment as the segment passes thescanning location; generating an electrical signal directly related tothe intensity of the light reflected from the segment; generatingelectrical pulses at a repetition rate proportional to the speed atwhich the workpiece is conveyed past the scanning location; and countingthe electrical pulses when the reflected light signal indicates that thelight beam is reflected from the altered portion, to thereby generatethe derived signal.
 3. The method of claim 1 wherein the altered portionis a surface layer that decreases in area during etching, the variableprocess parameter is the rate of etchant flow, and the adjusting stepfurther comprises: increasing the rate of etchant flow when the derivedsignal is greater than the predetermined signal, and decreasing the rateof etchant flow when the derived signal is less than the predeterminedsignal.
 4. The method of claim 1 wherein the altered portion is asurface layer that decreases in area during etching, the variableprocess parameter is the speed at which the workpiece is conveyedthrough the etching process and the adjusting step further comprises:increasing the conveying speed when the derived signal is less than thepredetermined signal, and decreasing the conveying speed when thederived signal is greater than the predetermined signal.
 5. The methodof claim 1 wherein the altered Portion is exposed substrate thatincreases in area during etching, the variable process parameter is therate of etchant flow, and the adjusting step further comprises:increasing the rate of etchant flow when the derived signal is less thanthe predetermined signal, and decreasing the rate of etchant flow whenthe derived signal is greater than the predetermined signal.
 6. Themethod of claim 1 wherein the altered portion is exposed substrate thatincreases in area during etching, the variable process parameter is thespeed at which the workpiece is conveyed through the etching process andthe adjusting step further comprises: increasing the conveying speedwhen the derived signal is greater than the predetermined signal, anddecreasing the conveying speed when the derived signal is less than thepredetermined signal.
 7. In an etching process, a method of determiningthe degree of etching of a workpiece having a surface layer portion thatis decreased in area during etching, which comprises: scanning a segmentof the workpiece, at a location in the etching process where the surfacelayer portion is being decreased in area, to derive a signalrepresenting the area of the surface layer portion within the segment;and comparing the derived signal with a predetermined signalrepresenting the desired area of the surface layer portion at thescanning location to indicate that, at the scanning location, theworkpiece is underetched when the derived signal is greater than thepredetermined signal, properly etched when the derived signalsubstantially equals the predetermined signal, and overetched when thederived signal is less than the predetermined signal.
 8. In an etchingprocess, a method of determining the degree of etching of a workpiecehaving an exposed substrate portion that is increased in area duringetching, which comprises: scanning a segment of the workpiece, at alocation in the etching process where the exposed substrate portion isbeing increased in area, to derive a signal representing the area of theexposed substrate portion within the segment; and comparing the derivedsignal with a predetermined signal representing the desired area of theexposed substrate portion at the scanning location to indicate that, atthe scanning location, the workpiece is overetched when the derivedsignal is greater than the predetermined signal, properly etched whenthe derived signal substantially equals the predetermined signal, andunderetched when the derived signal is less than the predeterminedsignal.
 9. Apparatus for regulating the degree of etching of a workpiececonveyed through an etching process having at least one variable processparameter affecting the degree of etching, the workpiece having aportion whose area is altered during etching, which comprises: means forscanning a segment of the workpiece, at a location in the etchingprocess where the portion is being altered, to determine a valuerepresenting the area of the altered portion within the segment; meansfor comparing the determined value with a standard value for thesegment; and means for adjusting a process parameter, according to theoutput of the comparison means, in a direction tending to reduce thedifference between the determined value and the standard value.
 10. Theapparatus of claim 9 in which the workpiece is conveyed through theetching machine at a constant speed, the variable process parameter isthe rate of etchant flow, and the means for scanning comprises:photodetector means, positioned at the scanning location to receivelight reflected from the segment, for generating an electrical signaldirectly related to the intensity of the reflected light; adiscriminator connected to the photodetector for generating an outputsignal when the signal from the photodetector is within a rangeindicating that the light is reflected from the altered portion of thesegment; a pulse generator for generating pulsEs at a constantrepetition rate; and a counter, connected to both the pulse generatorand the discriminator, for counting the pulses from the pulse generatorwhen the output signal from the discriminator is present, the countaccumulated in the counter being the value representing the area of thealtered portion within the segment.
 11. The apparatus of claim 9 inwhich the rate of etchant flow in the etching process is constant, thevariable process parameter is the speed at which the workpiece isconveyed through the etching process, and the means for scanningcomprises: photodetector means, positioned at the scanning location toreceive light reflected from the segment, for generating an electricalsignal directly related to the intensity of the reflected light; adiscriminator connected to the photodetector for activating an outputsignal when the signal from the photodetector is within a rangeindicating that the light is reflected from the altered portion of thesegment; means for generating pulses at a repetition rate proportionalto the conveying speed; and a counter, connected to both the pulsegenerating means and the discriminator, for counting the pulses from thepulse generating means when the output signal from the discriminator ispresent, the count accumulated in the counter being the valuerepresenting the area of the altered portion within the segment. 12.Apparatus for determining the degree of etching of a workpiece in anetching process, the workpiece having a surface layer portion that isdecreased in area during etching, which comprises: means for scanning asegment of the workpiece, at a location in the etching process where thesurface layer portion is being decreased in area, to determine a valuerepresenting the area of the surface layer portion within the segment;means for comparing the determined value with a standard value for thesegment; and means for indicating that the workpiece is overetched whenthe determined value is less than the standard value, properly etchedwhen the determined value substantially equals the standard value, andunderetched when the determined value is greater than the standardvalue.
 13. Apparatus for determining the degree of etching of aworkpiece in an etching process, the workpiece having an exposedsubstrate portion that is increased in area during etching, whichcomprises: means for scanning a segment of the workpiece, at a locationin the etching process where the exposed substrate portion is beingincreased in area, to determine a value representing the area of theexposed substrate portion within the segment; means for comparing thedetermined value with a standard value for the segment; and means forindicating that the workpiece is overetched when the determined value isgreater than the standard value, properly etched when the determinedvalue substantially equals the standard value, and underetched when thedetermined value is less than the standard value.
 14. A method ofcontrolling the selective removal of a layer of material from aworkpiece comprising the steps of: subjecting the workpiece to avariable rate etching process, scanning a segment of the workpiece at anintermediate point in the process to derive a measure of the portion ofthe segment from which the material has been removed. comparing thederived measure with a standard measure representing the desired amountof material removal at said intermediate point, and varying the etchingrate in accordance with the comparison at said intermediate point.